Bioprostheses manufactured from porcine aortic valves fixed with glutaraldehyde under various conditions of pressure were examined by polarized light microscopy to evaluate the degree of waviness or "crimping" of the collagen in the leaflets. Results obtained showed that the collagen crimping was present in all valves fixed at very low pressure but disappeared with increasing sensation pressure. Preservation of the crimping of collagen is considered important in maximizing the durability of bioprosthetic valves.